I got a poster in the mail that I ordered online, and it came rolled up, and slightly creased on one end of the roll (the end was flattened out into a sort of lemon shape, if you know what I mean). It wasn’t completely squashed, but enough that it holds its new shape; when it’s unrolled there are a series of horizontal crease-markings down the right side. Is there any way I can fix or reduce the creasing? I was hoping to get this framed, but can’t buy another…
Thanks!
What kind of poster is it? The glossy kind?
In the conservation lab we flatten paper in a humidification chamber, but people don’t have them lying around and they work best on non-coated papers.
What you can do is put your iron on low heat and gently try to press your poster. I’d try it with another piece of paper or a thin press cloth between the iron and the poster. Do not use steam. That’s how I get the wrinkles out of sewing patterns, but I’d be a little worried about the coated paper. Do it from the back and start at the lowest heat setting possible, and if it dosen’t work go up just a teensy bit. Be careful not to scorch it.
::pokes head into thread to see if there’s any risk of becoming a creased poster, and how to avoid it if it proves to be a significant concern::
Just don’t roll yourself up and ship yourself in a flimsy box, and you’ll do just fine 
Zsofia: thanks for the advice! I’ll try it later tonight…
Take it to a framing shop (Ben Franklin and Hobby Lobby stores usually have one) and ask them to dry-mount it for you. They will mount it on a piece of foamboard, which will flatten it out about as much as you’re going to get, and probably still reasonalbly presentable. Mr. S has a map of Middle Earth that was folded in four pieces; we had it dry-mounted and while you can still see the fold marks, it looks OK.
Dry-mounting can be icky from a conservation standpoint. I mean, it dosen’t really matter for most things you want to hang on your wall, but it isn’t archival in the long run. The interior of foam-core is acidic. It’s up to you if this matters to you, of course.
If you’re in or near a large city or university, call the library and ask for someone who does “conservation” of books and maps. Such a person will have the best advice for you, Garnet.
If that’s not feasible, I’d suggest following Zsofia’s advice, putting a sheet of waxed paper, rather than cloth or plain paper between the poster and the iron. And turn the poster over, first. 
See how that turns out and report back? If it’s insufficient, and there’s no visible adherence of the wax to the back, you can try it on the front.
The main consideration is that you’re most probably dealing with what is called “clay-coat” paper (what the poster is printed on).
State archives are another often-overlooked resource for paper and book conservation. Part of their mission is often to assist the state’s citizens with their own conservation problems.
Put a towel over it and run an iron over it.
Addressing the larger problem: It was damaged in shipping. That should be the seller’s problem to fix, not the buyer. Every poster I’ve ordered has had a lot of effort put into keeping it rolled properly, so that implies they are taking responsibility if it’s creased. As an analogy, if you bought a coffee mug, and it arrived cracked, would you be asking here about how to glue together a mug?